Network Working Group P. Gutmann
Internet-Draft University of Auckland
Intended status: Standards Track November 24, 2016
Expires: May 28, 2017
Simple Certificate Enrolment Protocol
draft-gutmann-scep-05
Abstract
This document specifies the Simple Certificate Enrolment Protocol
(SCEP), a PKI protocol that leverages existing technology by using
CMS (formerly known as PKCS #7) and PKCS #10 over HTTP. SCEP is the
evolution of the enrolment protocol sponsored by Cisco Systems, which
enjoys wide support in both client and server implementations, as
well as being relied upon by numerous other industry standards that
work with certificates.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 28, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 4
2. SCEP Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. SCEP Entities . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Client . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2. Certificate Authority . . . . . . . . . . . . . . . . 5
2.1.3. CA Certificate Distribution . . . . . . . . . . . . . 5
2.2. Client authentication . . . . . . . . . . . . . . . . . . 6
2.3. Enrolment authorization . . . . . . . . . . . . . . . . . 7
2.4. Certificate Enrolment/Renewal/Update . . . . . . . . . . 7
2.4.1. Client State Transitions . . . . . . . . . . . . . . 8
2.5. Certificate Access . . . . . . . . . . . . . . . . . . . 10
2.6. CRL Access . . . . . . . . . . . . . . . . . . . . . . . 10
2.7. Certificate Revocation . . . . . . . . . . . . . . . . . 11
2.8. Mandatory-to-Implement Functionality . . . . . . . . . . 11
3. SCEP Secure Message Objects . . . . . . . . . . . . . . . . . 11
3.1. SCEP Message Object Processing . . . . . . . . . . . . . 13
3.2. SCEP pkiMessage . . . . . . . . . . . . . . . . . . . . . 13
3.2.1. Signed Transaction Attributes . . . . . . . . . . . . 13
3.2.1.1. transactionID . . . . . . . . . . . . . . . . . . 15
3.2.1.2. messageType . . . . . . . . . . . . . . . . . . . 15
3.2.1.3. pkiStatus . . . . . . . . . . . . . . . . . . . . 16
3.2.1.4. failInfo . . . . . . . . . . . . . . . . . . . . 16
3.2.1.5. senderNonce and recipientNonce . . . . . . . . . 17
3.2.2. SCEP pkcsPKIEnvelope . . . . . . . . . . . . . . . . 17
3.3. SCEP pkiMessage types . . . . . . . . . . . . . . . . . . 17
3.3.1. PKCSReq/RenewalReq/UpdateReq . . . . . . . . . . . . 17
3.3.2. CertRep . . . . . . . . . . . . . . . . . . . . . . . 18
3.3.2.1. CertRep SUCCESS . . . . . . . . . . . . . . . . . 18
3.3.2.2. CertRep FAILURE . . . . . . . . . . . . . . . . . 19
3.3.2.3. CertRep PENDING . . . . . . . . . . . . . . . . . 19
3.3.3. CertPoll (GetCertInitial) . . . . . . . . . . . . . . 19
3.3.4. GetCert . . . . . . . . . . . . . . . . . . . . . . . 20
3.3.5. GetCRL . . . . . . . . . . . . . . . . . . . . . . . 21
3.4. Degenerate certificates-only CMS Signed-Data . . . . . . 21
3.5. CA Capabilities . . . . . . . . . . . . . . . . . . . . . 21
3.5.1. GetCACaps HTTP Message Format . . . . . . . . . . . . 21
3.5.2. CA Capabilities Response Format . . . . . . . . . . . 22
4. SCEP Transactions . . . . . . . . . . . . . . . . . . . . . . 24
4.1. HTTP POST and GET Message Formats . . . . . . . . . . . . 24
4.2. Get CA Certificate . . . . . . . . . . . . . . . . . . . 25
4.2.1. Get CA Certificate Response Message Format . . . . . 25
4.2.1.1. CA Certificate Response Message Format . . . . . 25
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4.2.1.2. CA Certificate Chain Response Message Format . . 25
4.3. Certificate Enrolment/Renewal/Update . . . . . . . . . . 26
4.3.1. Certificate Enrolment/Renewal/Update Response Message 26
4.4. Poll for Client Initial Certificate . . . . . . . . . . . 27
4.4.1. Polling Response Message Format . . . . . . . . . . . 27
4.5. Certificate Access . . . . . . . . . . . . . . . . . . . 27
4.5.1. Certificate Access Response Message Format . . . . . 28
4.6. CRL Access . . . . . . . . . . . . . . . . . . . . . . . 28
4.6.1. CRL Access Response Message Format . . . . . . . . . 28
4.7. Get Next Certificate Authority Certificate . . . . . . . 28
4.7.1. Get Next CA Response Message Format . . . . . . . . . 28
5. SCEP State Transitions . . . . . . . . . . . . . . . . . . . 29
5.1. Successful Transactions . . . . . . . . . . . . . . . . . 29
5.2. Transactions with Errors . . . . . . . . . . . . . . . . 30
6. Contributors/Acknowledgements . . . . . . . . . . . . . . . . 33
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
8. Security Considerations . . . . . . . . . . . . . . . . . . . 33
8.1. General Security . . . . . . . . . . . . . . . . . . . . 33
8.2. Use of the CA keypair . . . . . . . . . . . . . . . . . . 33
8.3. Challenge Password . . . . . . . . . . . . . . . . . . . 34
8.4. Lack of Certificate Issue Confirmation . . . . . . . . . 34
8.5. GetCACaps Issues . . . . . . . . . . . . . . . . . . . . 34
8.6. Lack of PoP in Renewal and Update Requests . . . . . . . 34
8.7. Unnecessary cryptography . . . . . . . . . . . . . . . . 35
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.1. Normative References . . . . . . . . . . . . . . . . . . 35
9.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Background Notes . . . . . . . . . . . . . . . . . . 36
Appendix B. Sample SCEP Messages . . . . . . . . . . . . . . . . 39
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
X.509 certificates serve as the basis for several standards-based
security protocols such as TLS [16], S/MIME [13], and IKE/IPsec [12].
When an X.509 certificate is issued there typically is a need for a
certificate management protocol to enable a PKI client to request a
certificate, certificate renewal, or certificate update from a
Certificate Authority (CA).
This specification defines a protocol, Simple Certificate Enrolment
Protocol (SCEP), for certificate management and certificate and CRL
queries. While widely deployed, this protocol omits some certificate
management features, e.g. certificate revocation transactions, which
may enhance the security achieved in a PKI. The IETF protocol suite
currently includes two further certificate management protocols with
more comprehensive functionality, Certificate Management Protocol
(CMP) [10] and Certificate Management over CMS (CMC) [9].
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Environments that do not require interoperability with SCEP
implementations MAY consider using the above-mentioned certificate
management protocols, however anyone considering this step should be
aware that the high level of complexity of these two protocols has
resulted in serious interoperability problems and corresponding lack
of industry support. SCEP's simplicity, while being a drawback in
terms of its slightly restricted functionality, also makes deployment
relatively straightforward, so that it enjoys widespread support and
ready interoperability across a range of platforms. While
implementers are encouraged to investigate one of the more
comprehensive alternative certificate management protocols in
addition to the protocol defined in this specification, anyone
wishing to deploy them should proceed with caution, and consider
support and interoperability issues before committing to their use.
The SCEP protocol supports the following general operations:
o CA public key distribution.
o Certificate enrolment and issue.
o Certificate renewal/update.
o Certificate query.
o CRL query.
SCEP makes extensive use of CMS [3] and PKCS #10 [6].
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
2. SCEP Overview
This section provides a high level overview of the functionality of
SCEP.
2.1. SCEP Entities
The entity types defined in SCEP are a client requesting a
certificate and a Certificate Authority (CA) that issues the
certificate. These are described in the following sections.
2.1.1. Client
A client MUST have the following information locally configured:
1. The CA fully qualified domain name or IP address.
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2. The CA HTTP CGI script path (this usually has a default value,
see Section 4.1).
3. The identifying information that is used for authentication of
the CA in Section 4.2.1, typically a certificate fingerprint.
2.1.2. Certificate Authority
A SCEP CA is the entity that signs client certificates. A CA MAY
enforce any arbitrary policies and apply them to certificate
requests, and MAY reject any request.
Since the client is expected to perform signature verification and
optionally encryption using the CA certificate, the keyUsage
extension in the CA certificate MUST indicate that it is valid for
digitalSignature and keyEncipherment (if required) alongside the
usual CA usages of keyCertSign and/or cRLSign.
2.1.3. CA Certificate Distribution
If the CA certificate(s) have not previously been acquired by the
client through some other means, the client MUST retrieve the them
before any PKI operation (Section 3) can be started. Since no public
key has yet been exchanged between the client and the CA, the
messages cannot be secured using CMS, and the data is instead
transferred in the clear.
If an intermediate CA is in use, a certificates-only CMS Signed-Data
message with a certificate chain consisting of all CA certificates is
returned. Otherwise the CA certificate itself is returned. The
transport protocol (Section 4) MUST indicate which one is returned.
The CA certificate MAY be provided out-of-band to the client.
Alternatively, the CA certificate fingerprint MAY be used to
authenticate a CA Certificate distributed by the GetCACert response
(Section 4.2) or via HTTP certificate-store access [11]. The
fingerprint is created by calculating a SHA-256 hash over the whole
CA certificate (for legacy reasons, a SHA-1 hash may be used by some
implementations).
After the client gets the CA certificate, it SHOULD authenticate the
certificate by comparing its fingerprint with the locally configured,
out-of-band distributed, identifying information. Intermediate CA
certificates, if any, are signed by a higher-level CA so there is no
need to authenticate them against the out-of-band data. Clients
SHOULD verify intermediate-level CA certificate signatures using the
issuing CA's certificate before use during protocol exchanges.
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Because a long time can pass between queries from a client to a CA
and because intermediate CA certificates can change over time, it is
recommended that a client not store intermediate CA certificates.
Instead, the client SHOULD retrieve them before each operation.
2.2. Client authentication
As with every protocol that uses public-key cryptography, the
association between the public keys used in the protocol and the
identities with which they are associated must be authenticated in a
cryptographically secure manner. The communications between the
client and the CA are secured using SCEP Secure Message Objects as
explained in Section 3, which specifies how CMS is used to encrypt
and sign the data. In order to perform the signing operation the
client uses an appropriate local certificate:
1. If the client does not have an appropriate existing certificate
then a locally generated self-signed certificate MUST be used.
The keyUsage extension in the certificate MUST indicate that it
is valid for digitalSignature and keyEncipherment (if required).
The self-signed certificate SHOULD use the same subject name as
in the PKCS #10 request. In this case the messageType is
PKCS10Req (see Section 3.2.1.2).
2. If the requesting system already has a certificate issued by the
SCEP CA, and the CA supports renewal (see Section 2.4), that
certificate SHOULD be used. In this case the messageType is
RenewalReq (see Section 3.2.1.2).
3. If the requesting system has no certificate issued by the new CA,
but has credentials from an alternate CA the certificate issued
by the alternate CA MAY be used. Policy settings on the new CA
will determine if the request can be accepted or not. This is
useful when enrolling with a new administrative domain using a
certificate from the old domain as credentials. In this case the
messageType is UpdateReq (see Section 3.2.1.2).
Note that although the above text describes three different types of
operations, in practice most implementations always apply the first
one even if an existing certificate already exists. For this reason
support for the first case is mandatory while support for the latter
two are optional (see Section 2.8).
During the certificate enrolment process, the client MUST use the
selected certificate's key when signing the CMS envelope (see
Section 3). This certificate will be either the self-signed one
matching the PKCS #10 request or the CA-issued one used to authorise
a renewal or update. If the key being certified allows encryption
then the CA's CertResp will use the same certificate's public key
when encrypting the response.
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Note that this means that, in the case of renewal and update
operations, the request is signed with, and the returned response may
be encrypted with, the key in the previously-issued certificate used
to authenticate the request, not the key in the PKCS #10 request.
This has security implications, see Section 8.6.
2.3. Enrolment authorization
PKCS #10 [6] specifies a PKCS #9 [5] challengePassword attribute to
be sent as part of the enrolment request. When utilizing the
challengePassword, the CA distributes a shared secret to the client
which will uniquely associate the enrolment request with the client.
Inclusion of the challengePassword by the SCEP client is OPTIONAL and
allows for unauthenticated authorization of enrolment requests
(which, however, requires manual approval of each certificate issue,
see below), or for renewal or update requests that are authenticated
by being signed with an existing certificate. The CMS envelope
protects the privacy of the challengePassword.
A client that is performing certificate renewal or update as per
Section 2.4 SHOULD omit the challengePassword but MAY send the
originally distributed password in the challengePassword attribute.
In the former case the SCEP CA MUST authenticate the request based on
the certificate used to sign the renewal or update request. In the
latter case the SCEP CA MAY use either the challengePassword or the
previously issued certificate (or both), depending on CA policy, to
authenticate the request. The SCEP CA MUST NOT attempt to
authenticate a client based on a self-signed certificate unless it
has been verified through out-of-band means such as a certificate
fingerprint.
To perform the authorization in manual mode the client's request is
placed in the PENDING state until the CA operator authorizes or
rejects it. Manual authorization is used when the client has only a
self-signed certificate that hasn't been previously authenticated by
the CA and/or a challengePassword is not available. The SCEP CA MAY
either reject unauthorized requests or mark them for manual
authorization according to CA policy.
2.4. Certificate Enrolment/Renewal/Update
A client starts an enrolment transaction (Section 3.3.1) by creating
a certificate request using PKCS #10 and sends it to the CA enveloped
using CMS (Section 3).
If the CA supports certificate renewal or update and if the CA policy
permits then a new certificate with new validity dates can be issued
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even though the old one is still valid. The CA MAY automatically
revoke the old client certificate. To renew or update an existing
certificate, the client uses the RenewalReq or UpdateReq message (see
Section 3.3) and signs it with the existing client certificate. The
client SHOULD use a new keypair when requesting a new certificate,
but MAY request a new certificate using the old keypair.
If the CA returns a CertRep message (Section 3.3.2) with status set
to PENDING, the client enters into polling mode by periodically
sending a CertPoll message (Section 3.3.3) to the CA until the CA
operator completes the manual authentication (approving or denying
the request).
If polling mode is being used then the client will send a single
PKCSReq/RenewalReq/UpdateReq message (Section 3.3.1), followed by 0
or more CertPoll messages (Section 3.3.3). The CA will in return
send 0 or more CertRep messages (Section 3.3.2) with status set to
PENDING in response to CertPolls, followed by a single CertRep
message (Section 3.3.2) with status set to either SUCCESS or FAILURE.
2.4.1. Client State Transitions
The client state transitions during the SCEP process are indicated in
Figure 1.
CertPoll
+-----<----+
| |
| | CertRep(PENDING)
| |
[CERT-NONEXISTENT] ------> [CERT-REQ-PENDING] ---------> [CERT-ISSUED]
^ PKCSReq | CertRep(SUCCESS)
| RenewalReq |
| UpdateReq |
| |
+-----------------------+
CertRep(FAILURE) or
Max-time/max-polls exceeded
Figure 1: State Transition Diagram
The certificate issue process starts at the state CERT-NONEXISTENT.
Sending a PKCSReq/RenewalReq/UpdateReq message changes the state to
CERT-REQ-PENDING.
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If the CA returns a CertRep message with pkiStatus set to SUCCESS
then the state changes to CERT-ISSUED.
If the CA returns a CertRep message with pkiStatus set to FAILURE or
there is no response then the state reverts back to CERT-NONEXISTENT.
If the CA returns a CertRep message with pkiStatus set to PENDING
then the client will keep polling by sending a CertPoll message until
either a CertRep message with status set to SUCCESS or FAILURE is
received or a timeout occurs or the maximum number of polls has been
exceeded.
A successful transaction in automatic mode:
CLIENT CA SERVER
PKCSReq: PKI cert. enrolment message
--------------------------------> CertRep: pkiStatus = SUCCESS
Certificate attached
<------------------------------
Receive issued certificate.
A successful transaction in manual mode:
CLIENT CA SERVER
PKCSReq: PKI cert. enrolment message
--------------------------------> CertRep: pkiStatus = PENDING
<------------------------------
CertPoll: Polling message
--------------------------------> CertRep: pkiStatus = PENDING
<------------------------------
................ <Manual identity authentication> ...............
CertPoll: Polling message
--------------------------------> CertRep: pkiStatus = SUCCESS
Certificate attached
<------------------------------
Receive issued certificate.
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2.5. Certificate Access
A certificate query message is defined for clients to retrieve a copy
of their own certificate from the CA. It allows clients that do not
store their certificates locally to obtain a copy when needed. This
functionality is not intended to provide a general purpose
certificate access service, which may be achieved via HTTP
certificate-store access [11] or LDAP.
To query a certificate from the CA, a client sends a request
consisting of the certificate's issuer name and serial number. This
assumes that the client has saved the issuer name and the serial
number of the issued certificate from the previous enrolment
transaction. The transaction to query a certificate consists of one
GetCert (Section 3.3.4) message and one CertRep (Section 3.3.2)
message, as shown below.
CLIENT CA SERVER
GetCert: PKI certificate query message
-------------------------------> CertRep: pkiStatus = SUCCESS
Certificate attached
<-----------------------------
Receive the certificate.
2.6. CRL Access
SCEP clients MAY request a CRL via one of three methods:
1. If the CA supports CRL Distribution Points (CRLDPs) [7], then the
CRL MAY be retrieved via the mechanism specified in the CRDLP.
2. If the CA supports HTTP certificate-store access [11], then the
CRL MAY be retrieved via the AuthorityInfoAcces [7] location
specified in the certificate.
3. Only if the CA does not support CRDLPs or HTTP access should a
CRL query be composed by creating a GetCRL message consisting of
the issuer name and serial number from the certificate whose
revocation status is being queried.
The CA SHOULD NOT support the GetCRL method because it does not scale
well due to the unnecessary cryptography (see Section 8.7) and it
requires the CA to be a high-availability service.
The message is sent to the SCEP CA in the same way as the other SCEP
requests. The transaction to retrieve a CRL consists of one GetCRL
PKI message and one CertRep PKI message, which contains only the CRL
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(no certificates) in a degenerate certificates-only CMS Signed-Data
message (Section 3.4), as shown below.
CLIENT CA SERVER
GetCRL: PKI CRL query message
---------------------------------->
CertRep: CRL attached
<-----------------------------
Receive the CRL
2.7. Certificate Revocation
SCEP does not specify a method to request certificate revocation. In
order to revoke a certificate, the client must contact the CA using a
non-SCEP defined mechanism.
2.8. Mandatory-to-Implement Functionality
At a minimum, all SCEP implementations compliant with this
specification MUST support GetCACaps (Section 3.5.1), GetCACert
(Section 4.2), PKCSReq (Section 3.3.1) (and its associated response
messages), communication of binary data via HTTP POST (Section 4.1),
and the AES and SHA-256 algorithms to secure pkiMessages
(Section 3.2).
For historical reasons implementations MAY support communications of
binary data via HTTP GET (Section 4.1), and the triple DES and SHA-1
algorithms to secure pkiMessages (Section 3.2). Implementations
SHOULD NOT support the obsolete and/or insecure single DES and MD5
algorithms used in earlier versions of this specification.
3. SCEP Secure Message Objects
CMS is a general enveloping mechanism that enables both signed and
encrypted transmission of arbitrary data. SCEP messages that require
confidentiality use two layers of CMS, as shown in Figure 2. By
applying both enveloping and signing transformations, the SCEP
message is protected both for the integrity of its end-to-end
transaction information and the confidentiality of its information
portion. Some messages do not require enveloping, in which case the
EnvelopedData in Figure 2 is omitted.
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pkiMessage {
contentType = signedData { pkcs-7 2 },
content {
digestAlgorithms,
encapsulatedContentInfo {
eContentType = data { pkcs-7 1 },
eContent { -- pkcsPKIEnvelope, optional
contentType = envelopedData { pkcs-7 3 },
content {
recipientInfo,
encryptedContentInfo {
contentType = data { pkcs-7 1 },
contentEncrAlgorithm,
encryptedContent {
messageData -- Typically PKCS #10 request
}
}
}
}
},
certificates, -- Optional
crls, -- Optional
signerInfo {
signedAttrs {
transactionID,
messageType,
pkiStatus,
failInfo, -- Optional
senderNonce / recipientNonce,
},
signature
}
}
}
Figure 2: CMS Layering
When a particular SCEP message carries data, this data is carried in
the messageData. CertRep messages will lack any signed content and
consist only of a pkcsPKIEnvelope (Section 3.2.2).
The remainder of this document will refer only to 'messageData', but
it is understood to always be encapsulated in the pkcsPKIEnvelope
(Section 3.2.2). The format of the data in the messageData is
defined by the messageType attribute (see Section 3.2) of the Signed-
Data. If there is no messageData to be transmitted, the entire
pkcsPKIEnvelope MUST be omitted.
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3.1. SCEP Message Object Processing
Creating a SCEP message consists of several stages. The content to
be conveyed (in other words the messageData) is first encrypted, and
the encrypted content is then signed.
The form of encryption to be applied depends on the capabilities of
the recipient's public key. If the key is encryption-capable (for
example RSA) then the messageData is encrypted using the recipient's
public key with the CMS KeyTransRecipientInfo mechanism. If the key
is not encryption-capable (for example ECDSA) then the messageData is
encrypted using the challengePassword with the CMS
PasswordRecipientInfo mechanism.
Once the messageData has been encrypted, it is signed with the
sender's public key. This completes the SCEP message that is then
sent to the recipient.
Note that some earlier implementations of this specification dealt
with non-encryption-capable keys by omitting the encryption stage,
based on the text in Section 3 that indicated that "the EnvelopedData
is omitted". This alternative processing mechanism SHOULD NOT be
used since it exposes the challengePassword used to authorise the
certificate issue.
3.2. SCEP pkiMessage
The basic building block of all secured SCEP messages is the SCEP
pkiMessage. It consists of a CMS Signed-Data content type. The
following restrictions apply:
o The eContentType in encapsulatedContentInfo MUST be data ({pkcs-7
1}).
o The signed content, if present (FAILURE and PENDING CertRep
messages will lack any signed content), MUST be a pkcsPKIEnvelope
(Section 3.2.2), and MUST match the messageType attribute.
o The SignerInfo MUST contain a set of authenticatedAttributes
(Section 3.2.1).
3.2.1. Signed Transaction Attributes
At a minimum, all messages MUST contain the following
authenticatedAttributes:
o A transactionID attribute (see Section 3.2.1.1).
o A messageType attribute (see Section 3.2.1.2).
o A fresh senderNonce attribute (see Section 3.2.1.5).
o Any attributes required by CMS.
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If the message is a CertRep, it MUST also include the following
authenticatedAttributes:
o A pkiStatus attribute (see Section 3.2.1.3).
o A failInfo attribute (see Section 3.2.1.4) if pkiStatus = FAILURE.
o A recipientNonce attribute (see Section 3.2.1.5) copied from the
senderNonce in the request that this is a response to.
The following transaction attributes are encoded as authenticated
attributes, and are carried in the SignerInfo for this Signed-Data.
+----------------+-----------------+--------------------------------+
| Attribute | Encoding | Comment |
+----------------+-----------------+--------------------------------+
| transactionID | PrintableString | Unique ID for this transaction |
| | | as a text string |
| | | |
| messageType | PrintableString | Decimal value as a numeric |
| | | text string |
| | | |
| pkiStatus | PrintableString | Decimal value as a numeric |
| | | text string |
| | | |
| failInfo | PrintableString | Decimal value as a numeric |
| | | text string |
| | | |
| senderNonce | OCTET STRING | Random nonce as a 16-byte |
| | | binary data string |
| | | |
| recipientNonce | OCTET STRING | Random nonce as a 16-byte |
| | | binary data string |
+----------------+-----------------+--------------------------------+
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The OIDs used for these attributes are as follows:
+-------------------+-----------------------------------------------+
| Name | ASN.1 Definition |
+-------------------+-----------------------------------------------+
| id-VeriSign | OBJECT_IDENTIFIER ::= {2 16 US(840) 1 |
| | VeriSign(113733)} |
| | |
| id-pki | OBJECT_IDENTIFIER ::= {id-VeriSign pki(1)} |
| | |
| id-attributes | OBJECT_IDENTIFIER ::= {id-pki attributes(9)} |
| | |
| id-transactionID | OBJECT_IDENTIFIER ::= {id-attributes |
| | transactionID(7)} |
| | |
| id-messageType | OBJECT_IDENTIFIER ::= {id-attributes |
| | messageType(2)} |
| | |
| id-pkiStatus | OBJECT_IDENTIFIER ::= {id-attributes |
| | pkiStatus(3)} |
| | |
| id-failInfo | OBJECT_IDENTIFIER ::= {id-attributes |
| | failInfo(4)} |
| | |
| id-senderNonce | OBJECT_IDENTIFIER ::= {id-attributes |
| | senderNonce(5)} |
| | |
| id-recipientNonce | OBJECT_IDENTIFIER ::= {id-attributes |
| | recipientNonce(6)} |
+-------------------+-----------------------------------------------+
The attributes are detailed in the following sections.
3.2.1.1. transactionID
A PKI operation is a transaction consisting of the messages exchanged
between a client and the CA. The transactionID is a text string
generated by the client when starting a transaction. The client MUST
generate a unique string as the transaction identifier, encoded as a
PrintableString, which MUST be used for all PKI messages exchanged
for a given operation such as a certificate issue.
3.2.1.2. messageType
The messageType attribute specifies the type of operation performed
by the transaction. This attribute MUST be included in all PKI
messages. The following message types are defined:
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o CertRep ("3") -- Response to certificate or CRL request.
o RenewalReq ("17") -- PKCS #10 certificate request for renewal of
an existing certificate.
o UpdateReq ("18") -- PKCS #10 certificate request for update of a
certificate issued by a different CA.
o PKCSReq ("19") -- PKCS #10 certificate request.
o CertPoll ("20") -- Certificate polling in manual enrolment.
o GetCert ("21") -- Retrieve a certificate.
o GetCRL ("22") -- Retrieve a CRL.
Undefined message types MUST BE treated as an error.
3.2.1.3. pkiStatus
All response messages MUST include transaction status information,
which is defined as a pkiStatus attribute:
o SUCCESS ("0") -- Request granted.
o FAILURE ("2") -- Request rejected. In this case the failInfo
attribute, as defined in Section 3.2.1.4, MUST also be present.
o PENDING ("3") -- Request pending for manual approval.
Undefined pkiStatus attributes MUST be treated as an error.
3.2.1.4. failInfo
The failInfo attribute MUST contain one of the following failure
reasons:
o badAlg ("0") -- Unrecognized or unsupported algorithm.
o badMessageCheck ("1") -- Integrity check (meaning signature
verification of the CMS message) failed.
o badRequest ("2") -- Transaction not permitted or supported.
o badTime ("3") -- The signingTime attribute from the CMS
authenticatedAttributes was not sufficiently close to the system
time (this failure code is present for legacy reasons and is
unlikely to be encountered in practice).
o badCertId ("4") -- No certificate could be identified matching the
provided criteria.
[Question: Is there any demand for a free-form UTF8String
attribute to explain what really went wrong? Trying to sort
out an error when all you ever get back is the near-universal
badRequest is almost impossible, adding a failInfoText
attribute to address this could be quite useful since it
would allow expressing information such as a failure to meet
CA policy, or indeed anything more complex than "no go away"].
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Undefined failInfo attributes are treated as an error.
3.2.1.5. senderNonce and recipientNonce
The senderNonce and recipientNonce attributes are a 16 byte random
number generated for each transaction. These are intended to prevent
replay attacks.
When a sender sends a PKI message to a recipient, a fresh senderNonce
MUST be included in the message. The recipient MUST copy the
senderNonce into the recipientNonce of the reply as a proof of
liveliness. The original sender MUST verify that the recipientNonce
of the reply matches the senderNonce it sent in the request. If the
nonce does not match, the message MUST be rejected.
Note that since SCEP exchanges consist of a single request followed
by a single response, the use of distinct sender and recipient nonces
is redundant since the client sends a nonce in its request and the CA
responds with the same nonce in its reply. In effect there's just a
single nonce, identified as senderNonce in the client's request and
recipientNonce in the CA's reply.
3.2.2. SCEP pkcsPKIEnvelope
The information portion of a SCEP message is carried inside an
EnvelopedData content type, as defined in CMS, with the following
restrictions:
o contentType in encryptedContentInfo MUST be data ({pkcs-7 1}).
o encryptedContent MUST be the SCEP message being transported (see
Section 4), and must match the messageType authenticated Attribute
in the pkiMessage.
3.3. SCEP pkiMessage types
All of the messages in this section are pkiMessages (Section 3.2),
where the type of the message MUST be specified in the 'messageType'
authenticated Attribute. Each section defines a valid message type,
the corresponding messageData formats, and mandatory authenticated
attributes for that type.
3.3.1. PKCSReq/RenewalReq/UpdateReq
The messageData for this type consists of a PKCS #10 Certificate
Request. The certificate request MUST contain at least the following
items:
o The subject Distinguished Name.
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o The subject public key.
o For a PKCSReq and if authorisation based on a password is being
used, a challengePassword attribute.
In addition to the authenticatedAttributes required for a valid CMS
message, the pkiMessage MUST include the following attributes:
o A transactionID attribute (Section 3.2.1.1).
o A messageType attribute (Section 3.2.1.2) set to PKCSReq,
RenewalReq, or UpdateReq as appropriate.
o A fresh senderNonce attribute (Section 3.2.1.5).
3.3.2. CertRep
The messageData for this type consists of a degenerate certificates-
only CMS Signed-Data message (Section 3.4). The exact content
required for the reply depends on the type of request that this
message is a response to. The request types are detailed in
Section 3.3.2.1 and in Section 4.
In addition to the authenticatedAttributes required for a valid CMS
message, this pkiMessage MUST include the following attributes:
o The transactionID attribute (Section 3.2.1.1) copied from the
request that this is a response to.
o A messageType attribute (Section 3.2.1.2) set to CertRep.
o A recipientNonce attribute (Section 3.2.1.5) copied from the
senderNonce in the request that this is a response to.
o A pkiStatus attribute (Section 3.2.1.3) set to the status of the
reply.
Earlier versions of this specification required that this message
include a senderNonce alongside the recipientNonce, which was to be
used to chain to subsequent polling operations. However if a single
message was lost during the potentially extended interval over which
polling could take place (see Section 5 for an example of this) then
if the implementation were to enforce this requirement the overall
transaction would fail even though nothing had actually gone wrong.
Because of this issue, implementations mostly ignored the requirement
to carry this nonce over to subsequent polling messages or to verify
its presence. Current versions of the specification no longer
require the chaining of nonces across polling operations.
3.3.2.1. CertRep SUCCESS
When the pkiStatus attribute is set to SUCCESS, the messageData for
this message consists of a degenerate certificates-only CMS Signed-
Data message (Section 3.4). The content of this degenerate
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certificates-only Signed-Data depends on what the original request
was, as outlined below.
+--------------+----------------------------------------------------+
| Request-type | Reply-contents |
+--------------+----------------------------------------------------+
| PKCSReq | The reply MUST contain at least the issued |
| | certificate in the certificates field of the |
| | Signed-Data. The reply MAY contain additional |
| | certificates, but the issued certificate MUST be |
| | the leaf certificate. |
| | |
| RenewalReq | Same as PKCSReq |
| | |
| UpdateReq | Same as PKCSReq |
| | |
| CertPoll | Same as PKCSReq |
| | |
| GetCert | The reply MUST contain at least the requested |
| | certificate in the certificates field of the |
| | Signed-Data. The reply MAY contain additional |
| | certificates, but the requested certificate MUST |
| | be the leaf certificate. |
| | |
| GetCRL | The reply MUST contain the CRL in the crls field |
| | of the Signed-Data. |
+--------------+----------------------------------------------------+
3.3.2.2. CertRep FAILURE
When the pkiStatus attribute is set to FAILURE, the reply MUST also
contain a failInfo (Section 3.2.1.4) attribute set to the appropriate
error condition describing the failure. The pkcsPKIEnvelope
(Section 3.2.2) MUST be omitted.
3.3.2.3. CertRep PENDING
When the pkiStatus attribute is set to PENDING, the pkcsPKIEnvelope
(Section 3.2.2) MUST be omitted.
3.3.3. CertPoll (GetCertInitial)
This message is used for certificate polling. For unknown reasons it
was referred to as "GetCertInitial" in earlier versions of this
specification. The messageData for this type consists of an
IssuerAndSubject:
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issuerAndSubject ::= SEQUENCE {
issuer Name,
subject Name
}
The issuer is set to the subjectName of the CA (in other words the
intended issuerName of the certificate that's being requested). The
subject is set to the subjectName used when requesting the
certificate.
Note that both of these fields are redundant, the CA is identified by
the recipientInfo in the pkcsPKIEnvelope (or in most cases simply by
the server that the message is being sent to) and the client/
transaction being polled is identified by the transactionID. Both of
these fields can be processed by the CA without having to unwrap and
process the issuerAndSubject. For this reason implementations SHOULD
assume that the polling operation will be controlled by the
recipientInfo and transactionID rather than the contents of the
messageData.
In addition to the authenticatedAttributes required for a valid CMS
message, this pkiMessage MUST include the following attributes:
o The same transactionID (Section 3.2.1.1) attribute from the
original PKCSReq/RenewalReq/UpdateReq message.
o A messageType attribute (Section 3.2.1.2) set to CertPoll.
o A fresh senderNonce attribute (Section 3.2.1.5).
3.3.4. GetCert
The messageData for this type consists of an IssuerAndSerialNumber as
defined in CMS which uniquely identifies the certificate being
requested. In addition to the authenticatedAttributes required for a
valid CMS message, this pkiMessage MUST include the following
attributes:
o A transactionID attribute (Section 3.2.1.1).
o A messageType attribute (Section 3.2.1.2) set to GetCert.
o A fresh senderNonce attribute (Section 3.2.1.5).
A self-signed certificate MAY be used in the signed envelope. This
enables the client to request their own certificate if they were
unable to store it previously.
This message type, while included here for completeness, applies
unnecessary cryptography and messaging overhead to the simple task of
transferring a certificate (see Section 8.7). Implementations SHOULD
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prefer HTTP certificate-store access [11] or LDAP over the use of
this message.
3.3.5. GetCRL
The messageData for this type consists of a IssuerAndSerialNumber as
defined in CMS containing the issuer name and serial number of the
certificate whose revocation status is being checked.
In addition to the authenticatedAttributes required for a valid CMS
message, this pkiMessage MUST include the following attributes:
o A transactionID attribute (Section 3.2.1.1).
o A messageType attribute (Section 3.2.1.2) set to GetCRL.
o A fresh senderNonce attribute (Section 3.2.1.5).
This message type, while included here for completeness, applies
unnecessary cryptography and messaging overhead to the simple task of
transferring a CRL (see Section 8.7). Implementations SHOULD prefer
HTTP certificate-store access [11] or LDAP over the use of this
message.
3.4. Degenerate certificates-only CMS Signed-Data
CMS includes a degenerate case of the Signed-Data content type in
which there are no signers. The use of such a degenerate case is to
disseminate certificates and CRLs. For SCEP the content field of the
ContentInfo value of a degenerate certificates-only Signed-Data MUST
be omitted.
When carrying certificates, the certificates are included in the
'certificates' field of the Signed-Data. When carrying a CRL, the
CRL is included in the 'crls' field of the Signed-Data.
3.5. CA Capabilities
In order to provide support for future enhancements to the protocol,
CAs MUST implement the GetCACaps message to allow clients to query
which functionality is available from the CA.
3.5.1. GetCACaps HTTP Message Format
This message requests capabilities from a CA, with the format:
"GET" CGI-PATH CGI-PROG "?operation=GetCACaps"
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with the message components as described in Section 4.
3.5.2. CA Capabilities Response Format
The response for a GetCACaps message is a list of CA capabilities, in
plain text, separated by <CR><LF> or <LF> characters, as follows
(quotation marks are NOT sent):
+--------------------+----------------------------------------------+
| Keyword | Description |
+--------------------+----------------------------------------------+
| "AES" | CA supports the AES encryption algorithm. |
| | |
| "DES3" | CA supports the triple DES encryption |
| | algorithm. |
| | |
| "GetNextCACert" | CA supports the GetNextCACert message. |
| | |
| "POSTPKIOperation" | CA supports PKIOPeration messages sent via |
| | HTTP POST. |
| | |
| "Renewal" | CA supports the Renewal CA operation. |
| | |
| "SHA-1" | CA supports the SHA-1 hashing algorithm. |
| | |
| "SHA-256" | CA supports the SHA-256 hashing algorithm. |
| | |
| "SHA-512" | CA supports the SHA-512 hashing algorithm. |
| | |
| "SCEPStandard" | CA supports all mandatory-to-implement |
| | sections of the SCEP standard. This keyword |
| | implies "AES", "POSTPKIOperation", and |
| | "SHA-256", as well as the provisions of |
| | Section 2.8. |
| | |
| "Update" | CA supports the Update CA operation. |
+--------------------+----------------------------------------------+
The client SHOULD use SHA-256 in preference to SHA-1 hashing and AES
in preference to triple DES if they are supported by the CA.
Although the CMS format allows any form of SHA-2 and AES to be
specified, in the interests of interoperability the de facto
universal standards of AES128-CBC and SHA-256 SHOULD be used.
Announcing some of these capabilities individually is redundant since
they're required as mandatory-to-implement functionality (see
Section 2.8) whose presence as a whole is signalled by the
"SCEPStandard" capability, but it may be useful to announce them in
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order to deal with old implementations that would otherwise default
to obsolete, insecure algorithms and mechanisms.
The CA MUST use the text case specified here, but clients SHOULD
ignore the text case when processing this message. Clients MUST
accept the standard HTTP-style <CR><LF>-delimited text as well as the
<LF>- delimited text specified in an earlier version of this
specification. A client MUST be able to accept and ignore any
unknown keywords that might be sent back by a CA.
If the CA supports none of the above capabilities it SHOULD return an
empty message. A CA MAY simply return an HTTP error. A client that
receives an empty message or an HTTP error SHOULD interpret the
response as if none of the requested capabilities are supported by
the CA.
(Note that at least one widely-deployed server implementation
supports several of the above operations but doesn't support the
GetCACaps message to indicate that it supports them. This means that
the equivalent of GetCACaps must be performed through server
fingerprinting, which can be done using the ID string "Microsoft-
IIS").
The Content-type of the reply SHOULD be "text/plain". Clients SHOULD
ignore the Content-type, as older implementations of SCEP may send
various Content-types.
Example:
GET /cgi-bin/pkiclient.exe?operation=GetCACaps
might return:
AES
GetNextCACert
POSTPKIOperation
SCEPStandard
SHA-256
This means that the CA supports modern crypto algorithms, the
GetNextCACert message, allows PKIOperation messages
(PKCSReq/RenewalReq/UpdateReq, GetCert, CertPoll, ...) to be sent
using HTTP POST, and is compliant with the final version of the SCEP
standard.
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4. SCEP Transactions
This section describes the SCEP Transactions and their HTTP [4]
transport mechanism.
4.1. HTTP POST and GET Message Formats
SCEP uses the HTTP "POST" and "GET" messages to exchange information
with the CA. The following defines the syntax of HTTP POST and GET
messages sent from a client to a CA:
"POST" CGI-PATH CGI-PROG "?operation=" OPERATION
"GET" CGI-PATH CGI-PROG "?operation=" OPERATION "&message=" MESSAGE
where:
o CGI-PATH defines the path to invoke the CGI program that parses
the request.
o CGI-PROG is set to be the string "pkiclient.exe". This is
intended to be the program that the CA will use to handle the SCEP
transactions.
o OPERATION depends on the SCEP transaction and is defined in the
following sections.
The CA will typically ignore CGI-PATH and/or CGI-PROG since it's
unlikely to be issuing certificates via a web server. Clients SHOULD
set CGI-PATH/CGI-PROG to the fixed string "/cgi-bin/pkiclient.exe"
unless directed to do otherwise to by the CA.
Early SCEP drafts performed all communications via "GET" messages,
including non-idempotent ones that should have been sent via "POST"
messages. This has caused problems because of the way that the
(supposedly) idempotent GET interacts with caches and proxies, and
because the extremely large GET requests created by encoding CMS
messages may be truncated in transit. These issues are typically not
visible when testing on a LAN, but crop up during deployment over
WANs. If the remote CA supports it, any of the CMS-encoded SCEP
messages SHOULD be sent via HTTP POST instead of HTTP GET. This
applies to any SCEP message except GetCACert, GetNextCACert, and
GetCACaps, and avoids the need for base64- and URL-encoding that's
required for GET messaging. The client can verify that the CA
supports SCEP messages via POST by looking for the "POSTPKIOperation"
capability (See Section 3.5.2).
If a client or CA uses HTTP GET and encounters HTTP-related problems
such as messages being truncated, seeing errors such as HTTP 414
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("Request URI too long"), or simply having the message not sent/
received at all, when standard requests to the server (for example
via a web browser) work, then this is a symptom of the problematic
use of HTTP GET. The solution to this problem is typically to move
to HTTP POST instead. In addition when using GET it's recommended to
test your implementation over the public internet from as many
locations as possible to determine whether the use of GET will cause
problems with communications.
When using GET messages to communicate binary data, base64 encoding
as specified in [2] MUST be used. The base64 encoded data is
distinct from "base64url" and may contain URI reserved characters,
thus it MUST be escaped as specified in [8] in addition to being
base64 encoded. Finally, the encoded data is inserted into the
MESSAGE portion of the HTTP GET request.
4.2. Get CA Certificate
To get the CA certificate(s), the client sends a GetCACert message to
the CA. The OPERATION MUST be set to "GetCACert". There is no
request data associated with this message.
4.2.1. Get CA Certificate Response Message Format
The response for GetCACert is different between the case where the CA
directly communicates with the client during the enrolment, and the
case where an intermediate CA exists and the client communicates with
this CA during the enrolment.
4.2.1.1. CA Certificate Response Message Format
If the CA does not have any intermediate CA certificates, the
response consists of a single X.509 CA certificate. The response
will have a Content-Type of "application/x-x509-ca-cert".
"Content-Type: application/x-x509-ca-cert"
<binary X.509>
4.2.1.2. CA Certificate Chain Response Message Format
If the CA has intermediate CA certificates, the response consists of
a degenerate certificates-only CMS Signed-Data message (Section 3.4)
containing the certificates, with the intermediate CA certificate(s)
as the leaf certificate(s). The response will have a Content-Type of
"application/x-x509-ca-ra-cert". Note that this designation is used
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for historical reasons due to its use in older versions of this
specification, no special meaning should be attached to the label.
"Content-Type: application/x-x509-ca-ra-cert"
<binary CMS>
4.3. Certificate Enrolment/Renewal/Update
A PKCSReq/RenewalReq/UpdateReq (Section 3.3.1) message is used to
perform a certificate enrolment, renewal, or update transaction. The
OPERATION MUST be set to "PKIOperation". Note that when used with
HTTP POST, the only OPERATION possible is "PKIOperation", so many CAs
don't check this value or even notice its absence. When implemented
using HTTP POST the message might look as follows:
POST /cgi-bin/pkiclient.exe?operation=PKIOperation HTTP/1.1
Content-Length: <length of data>
<binary CMS data>
When implemented using HTTP GET this might look as follows:
GET /cgi-bin/pkiclient.exe?operation=PKIOperation& \
message=MIAGCSqGSIb3DQEHA6CAMIACAQAxgDCBzAIBADB2MG \
IxETAPBgNVBAcTCE......AAAAAA== HTTP/1.1
4.3.1. Certificate Enrolment/Renewal/Update Response Message
If the request is granted, a CertRep message (Section 3.3.2) with
pkiStatus set to SUCCESS is returned. The reply MUST also contain
the certificate (and MAY contain any other certificates needed by the
client). The issued certificate MUST be the first in the list.
If the request is rejected, a CertRep message (Section 3.3.2) with
pkiStatus set to FAILURE is returned. The reply MUST also contain a
failInfo attribute.
If the the CA is configured to manually authenticate the client, a
CertRep message (Section 3.3.2) with pkiStatus set to PENDING MAY be
returned. The CA MAY return a PENDING for other reasons.
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The response will have a Content-Type of "application/x-pki-message".
"Content-Type: application/x-pki-message"
<binary CertRep message>
4.4. Poll for Client Initial Certificate
Whe the client receives a CertRep message with pkiStatus set to
PENDING, it will enter the polling state by periodically sending
CertPoll messages to the CA until either the request is granted and
the certificate is sent back or the request is rejected or some
preconfigured time limit for polling or maximum number of polls is
exceeded. The OPERATION MUST be set to "PKIOperation".
CertPoll messages exchanged during the polling period MUST carry the
same transactionID attribute as the previous PKCSReq/RenewalReq/
UpdateReq. A CA receiving a CertPoll for which it does not have a
matching PKCSReq/RenewalReq/UpdateReq MUST ignore this request.
Since at this time the certificate has not been issued, the client
can only use its own subject name (which was contained in the
original PKCS# 10 sent via PKCSReq/RenewalReq/UpdateReq) to identify
the polled certificate request (but see the note on identification
during polling in Section 3.3.3). In theory there can be multiple
outstanding requests from one client (for example, if different keys
and different key-usages were used to request multiple certificates),
so the transactionID must also be included to disambiguate between
multiple requests. In practice however it's safer for the client to
not have multiple requests outstanding at any one time, since this
tends to confuse some CAs.
4.4.1. Polling Response Message Format
The response messages for CertPoll are the same as in Section 4.3.1.
4.5. Certificate Access
A client can query an issued certificate from the SCEP CA, as long as
the client knows the issuer name and the issuer assigned certificate
serial number.
This transaction consists of one GetCert (Section 3.3.4) message sent
to the CA by a client, and one CertRep (Section 3.3.2) message sent
back from the CA. The OPERATION MUST be set to "PKIOperation".
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4.5.1. Certificate Access Response Message Format
In this case, the CertRep from the CA is same as in Section 4.3.1,
except that the CA will either grant the request (SUCCESS) or reject
it (FAILURE).
4.6. CRL Access
Clients can request a CRL from the SCEP CA as described in
Section 2.6. The OPERATION MUST be set to "PKIOperation".
4.6.1. CRL Access Response Message Format
The CRL is sent back to the client in a CertRep (Section 3.3.2)
message. The information portion of this message is a degenerate
certificates-only Signed-Data (Section 3.4) that contains only the
most recent CRL in the crls field of the Signed-Data.
4.7. Get Next Certificate Authority Certificate
When a CA certificate expires then, depending on client and CA
policy, certificates that have been signed by it may no longer be
regarded as valid. CA key rollover provides a mechanism by which the
CA MAY distribute a new CA certificate which is valid in the future
once the current certificate has expired. When a CA certificate is
about to expire, clients need to retrieve the CA's next CA
certificate (i.e. the rollover certificate). This is done via the
GetNextCACert message. The OPERATION MUST be set to "GetNextCACert".
There is no request data associated with this message.
Clients MAY store the not-yet-valid CA certificate, and any not-yet-
valid client certificates obtained, until such time that they are
valid, at which point clients switch over to using the newly valid
certificates (but see the note in Section 2.1.3 about storing CA
certificates).
4.7.1. Get Next CA Response Message Format
The response consists of a Signed-Data CMS message, signed by the
current CA signing key. Clients MUST validate the signature on the
message before accepting any of its contents. The response will have
a Content-Type of "application/x-x509-next-ca-cert".
"Content-Type: application/x-x509-next-ca-cert"
<binary CMS>
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The content of the Signed-Data message is a degenerate certificates-
only Signed-Data message (Section 3.4) containing the new CA
certificate(s) to be used when the current CA certificate expires.
If the CA does not have rollover certificate(s) it MUST reject the
request. It SHOULD also remove the GetNextCACert setting from the CA
capabilities returned by GetCACaps until it does have rollover
certificates.
5. SCEP State Transitions
The following section gives several examples of client to CA
transactions. Client actions are indicated in the left column, CA
actions are indicated in the right column, and the transactionID is
given in parentheses. The first transaction, for example, would read
like this:
"Client Sends PKCSReq message with transactionID 1 to the CA. The CA
signs the certificate and constructs a CertRep Message containing the
signed certificate with a transaction ID 1. The client receives the
message and installs the certificate locally".
5.1. Successful Transactions
Successful Enrolment Case: Automatic processing
PKCSReq (1) ----------> CA issues certificate
<---------- CertRep (1) SUCCESS
Client installs certificate
Successful Enrolment Case: Manual authentication required
PKCSReq (2) ----------> Cert request goes into queue
<---------- CertRep (2) PENDING
CertPoll (2) ----------> Still pending
<---------- CertRep (2) PENDING
CertPoll (2) ----------> CA issues certificate
<---------- CertRep (2) SUCCESS
Client installs certificate
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CA certificate rollover case:
GetNextCACert ---------->
<---------- New CA certificate
PKCSReq* ----------> CA issues certificate with
new key
<---------- CertRep SUCCESS
Client stores certificate
for installation when
existing certificate expires.
* Enveloped for the new CA certificate. The CA will use the envelope
to determine which key to use to issue the client certificate.
5.2. Transactions with Errors
In the case of polled transactions that aren't completed
automatically, there are two potential options for dealing with a
transaction that's interrupted due to network or software/hardware
issues. The first is for the client to preserve its transaction
state and resume the CertPoll polling when normal service is
restored. The second is for the client to begin a new transaction by
sending a new PKCSReq/RenewalReq/UpdateReq rather than continuing the
previous CertPoll. Both options have their own advantages and
disadvantages.
The CertPoll continuation requires that the client maintain its
transaction state for then time when it resumes polling. This is
relatively simple if the problem is a brief network outage, but less
simple when the problem is a client crash and restart. In addition
the CA may treat a lost network connection as the end of a
transaction, so that a new connection followed by a CertPoll will be
treated as an error.
The PKCSReq/RenewalReq/UpdateReq continuation doesn't require any
state to be maintained since it's a new transaction, however it may
cause problems on the CA side if the certificate was successfully
issued but the client never received it, since the resumed
transaction attempt will appear to be a request for a duplicate
certificate (see Section 8.4 for more on why this is a problem). In
this case the CA may refuse the transaction, or require manual
intervention to remove/revoke the previous certificate before the
client can request another one.
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Since the new-transaction resume is more robust in the presence of
errors and doesn't require special-case handling by either the client
or CA, clients SHOULD use the new-transaction option in preference to
the resumed-CertPoll option to recover from errors.
Resync Case 1: Client resyncs via new PKCSReq (recommended):
PKCSReq (3) ----------> Cert request goes into queue
<---------- CertRep (3) PENDING
CertPoll (3) ----------> Still pending
X-------- CertRep(3) PENDING
(Network outage)
(Client reconnects)
PKCSReq (4) ---------->
<---------- CertRep (4) PENDING
etc...
Resync Case 2: Client resyncs via resumed CertPoll after a network
outage (not recommended, use PKCSReq to resync):
PKCSReq (5) ----------> Cert request goes into queue
<---------- CertRep (5) PENDING
CertPoll (5) ----------> Still pending
X-------- CertRep(5) PENDING
(Network outage)
(Client reconnects)
CertPoll (5) ----------> CA issues certificate
<---------- CertRep (5) SUCCESS
Client installs certificate
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Resync Case 3: Special-case variation of case 2 where the CertRep
SUCCESS rather than the CertRep PENDING is lost (recommended):
PKCSReq (6) ----------> Cert request goes into queue
<---------- CertRep (6) PENDING
CertPoll (6) ----------> Still pending
<---------- CertRep (6) PENDING
CertPoll (6) ----------> CA issues certificate
X-------- CertRep(6) SUCCESS
(Network outage)
(Client reconnects)
PKCSReq (7) ----------> There is already a valid
certificate with this DN.
<---------- CertRep (7) FAILURE
Admin revokes certificate
PKCSReq (7) ----------> CA issues new certificate
<---------- CertRep (7) SUCCESS
Client installs certificate
Resync Case 4: Special-case variation of case 1 where the CertRep
SUCCESS rather than the CertRep PENDING is lost (not recommended, use
PKCSReq to resync):
PKCSReq (8) ----------> Cert request goes into queue
<---------- CertRep (8) PENDING
CertPoll (8) ----------> Still pending
<---------- CertRep (8) PENDING
CertPoll (8) ----------> CA issues certificate
X-------- CertRep(8) SIGNED CERT
(Network outage)
(Client reconnects)
CertPoll (8) ----------> Certificate already issued
<---------- CertRep (8) SUCCESS
Client installs certificate
As these examples indicate, resumption from an error via a resumed
CertPoll is tricky due to the state that needs to be held by both the
client and/or the CA. A PKCSReq/RenewalReq/UpdateReq resume is the
easiest to implement since it's stateless and is identical for both
polled and non-polled transactions, while a CertPoll resume treats
the two differently (a non-polled transaction is resumed with a
PKCSReq/RenewalReq/UpdateReq, a polled transaction is resumed with a
CertPoll). For this reason this specification recommends that error
recovery be handled via a new PKCSReq rather than a resumed CertPoll.
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6. Contributors/Acknowledgements
The editor would like to thank all of the previous editors, authors
and contributors: Cheryl Madson, Xiaoyi Liu, David McGrew, David
Cooper, Andy Nourse, Max Pritikin, Jan Vilhuber, and others for their
work maintaining the draft over the years. Numerous other people
have contributed during the long life cycle of the draft and all
deserve thanks. In addition several PKCS #7 / CMS libraries
contributed to interoperability by doing the right thing despite what
earlier SCEP drafts required.
The earlier authors would like to thank Peter William of ValiCert,
Inc. (formerly of VeriSign, Inc.) and Alex Deacon of VeriSign, Inc.
and Christopher Welles of IRE, Inc. for their contributions to early
versions of this protocol and this document.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
The security goal of SCEP is that no adversary can subvert the public
key/identity binding from that intended. An adversary is any entity
other than the client and the CA participating in the protocol.
This goal is met through the use of CMS and PKCS #10 encryption and
digital signatures using authenticated public keys. The CA's public
key is authenticated via out-of-band means such as the checking of
the CA fingerprint and the SCEP client's public key is authenticated
through manual or pre-shared secret authentication.
8.1. General Security
Common key-management considerations such as keeping private keys
truly private and using adequate lengths for symmetric and asymmetric
keys must be followed in order to maintain the security of this
protocol. This is especially true for CA keys which, when
compromised, compromise the security of all relying parties.
8.2. Use of the CA keypair
A CA key pair is generally meant for, and is usually flagged as,
being usable for certificate (and CRL) signing exclusively rather
than data signing or encryption. The SCEP protocol however uses the
CA private key to both sign and optionally encrypt CMS transport
messages. This is generally considered undesirable as it widens the
possibility of an implementation weakness and provides an additional
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location where the private key must be used (and hence is slightly
more vulnerable to exposure) and where a side-channel attack might be
applied.
8.3. Challenge Password
The challengePassword sent in the PKCS #10 enrolment request is
signed and encrypted by way of being encapsulated in a pkiMessage.
When saved by the CA, care should be taken to protect this password,
for example by storing a salted iterated hash of the password rather
than the password itself.
8.4. Lack of Certificate Issue Confirmation
SCEP provides no confirmation that the issued certificate was
successfully received and processed by the client. This means that
if the CertRep message is lost or can't be processed by the client
then the CA will consider the certificate successfully issued while
the client won't. If this situation is of concern then the correct
issuance of the certificate will need to be verified by out-of-band
means, for example through the client sending a message signed by the
newly-issued certificate to the CA. This also provides the proof of
possession that's not present in the case of an update or renewal
operation, see Section 8.6.
8.5. GetCACaps Issues
The GetCACaps response is not authenticated by the CA. This allows
an attacker to perform downgrade attacks on the cryptographic
capabilities of the client/CA exchange.
8.6. Lack of PoP in Renewal and Update Requests
Renewal and update operations (but not standard certificate-issue
operations) are processed via a previously-issued certificate and its
associated private key, not the key in the PKCS #10 request. This
means that a client no longer demonstrates proof of possession (PoP)
of the private key corresponding to the public key in the PKCS #10
request. It is therefore possible for a client to re-certify an
existing key used by a third party, so that two or more certificates
exist for the same key. By switching out the certificate in a
signature, an attacker can appear to have a piece of data signed by
their certificate rather than the original signer's certificate.
This, and other, attacks are described in S/MIME ESS [14].
Avoiding these types of attacks requires situation-specific measures.
For example CMS/SMIME implementations may use the ESSCertID attribute
from S/MIME ESS [14] or its successor S/MIME ESSv2 [15] to
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unambiguously identify the signing certificate, however other
mechanisms and protocols typically don't defend against this attack.
8.7. Unnecessary cryptography
Some of the SCEP exchanges use unnecessary signing and encryption
operations. In particular the GetCert and GetCRL exchanges are
encrypted and signed in both directions. The information requested
is public and thus encrypting the requests is of questionable value.
In addition CRLs and certificates sent in responses are already
signed by the CA and can be verified by the recipient without
requiring additional signing and encryption. More lightweight means
of retrieving certificates and CRLs such as HTTP certificate-store
access [11] and LDAP are recommended for this reason.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[3] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 5652, September 2009.
[4] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[5] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
November 2000.
[6] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
November 2000.
[7] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[8] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
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9.2. Informative References
[9] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, June 2008.
[10] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210, September 2005.
[11] Gutmann, P., "Internet X.509 Public Key Infrastructure
Operational Protocols: Certificate Store Access via HTTP",
RFC 4387, February 2006.
[12] Alighieri, D., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, March 1300.
[13] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[14] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[15] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035, August 2007.
[16] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Appendix A. Background Notes
This specification has spent more than sixteen years in the draft
stage. Its original goal, provisioning IPsec routers with
certificates, has long since changed to general device/embedded
system/IoT use. To fit this role, extra features were bolted on in a
haphazard manner through the addition of a growing list of appendices
and by inserting additional, often conflicting, paragraphs in various
locations in the body text. Since existing features were never
updated as newer ones were added, the specification accumulated large
amounts of historical baggage over time. If OpenPGP was described as
"a museum of 1990s crypto" then the SCEP draft was its graveyard.
About five years ago the specification, which even at that point had
seen only sporadic re-posts of the existing document, was more or
less abandoned by its original sponsors. Due to its widespread use
in large segments of the industry, the specification was rebooted in
2015, cleaning up fifteen years worth of accumulated cruft, fixing
errors, clarifying ambiguities, and bringing the algorithms and
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standards used into the current century (prior to the update, the de-
facto lowest-common denominator algorithms used for interoperability
were the forty-year-old single DES and broken MD5 hash algorithms).
Other changes include:
o Resolved contradictions in the text, for example a requirement
given as a MUST in one paragraph and a SHOULD in the next, a MUST
NOT in one paragraph and a MAY a few paragraphs later, a SHOULD
NOT contradicted later by a MAY, and so on.
o Merged several later fragmentary addenda placed in appendices (for
example the handling of certificate renewal and update) with the
body of the text.
o Merged the SCEP Transactions and SCEP Transport sections, since
the latter mostly duplicated (with occasional inconsistencies) the
former.
o Updated the algorithms to ones dating from at least this century.
o Did the same for normative references to other standards.
o Updated the text to use consistent terminology for the client and
CA rather than a mixture of client, requester, end entity, server,
certificate authority, certification authority, and CA.
o Corrected incorrect references to other standards, e.g.
IssuerAndSerial -> IssuerAndSerialNumber.
o Corrected errors such as a statement that when both signature and
encryption certificates existed, the signature certificate was
used for encryption.
o Condensed redundant discussions of the same topic spread across
multiple sections into a single location. For example the
description of intermediate CA handling previously existed in
three different locations, with slightly different reqirements in
each one.
o Added a description of how pkiMessages were processed, which was
never made explicit in the original specification. This led to
creative interpretations that had security problems but were
employed anyway due to the lack of specific guidance on what to
do.
o Relaxed some requirements that didn't serve any obvious purpose
and that major implementations didn't seem to be enforcing. For
example the requirement that the self-signed certificate used with
a request MUST contain a subject name that matched the one in the
PKCS #10 request was relaxed to a SHOULD because a number of
implementations either ignored the issue entirely or at worst
performed some minor action like creating a log entry after which
they continued anyway.
o Removed discussion of the transactionID from the security
considerations, since the instructions there were directly
contradicted by the discussion of the use of the transactionID in
Section 5.
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o Clarified sections that were unclear or even made no sense, for
example the requirement for a "hash on the public key" [sic]
encoded as a PrintableString.
o Renamed "RA certificates" to "intermediate CA certificates". The
original document at some point added mention of RA certificates
without specifying how the client was to determine that an RA was
in use, how the RA operations were identified in the protocol, or
how it was used. It's unclear whether what was meant was a true
RA or merely an intermediate CA, as opposed to the default
practice of having certificates issued directly from a single root
CA certificate. This update uses the term "intermediate CA
certificates", since this seems to have been the original intent
of the text.
o Redid the PKIMessage diagram to match what was specified in CMS,
the original diagram omitted a number of fields and nested data
structures which meant that the diagram didn't match either the
text or the CMS specification.
o Removed the requirement for a CertPoll to contain a
recipientNonce, since CertPoll is a client message and will never
be sent in response to a message containing a senderNonce. See
also the note in Section 3.3.2.
o Clarified certificate renewal and update. These represent a
capability that was bolted onto the original protocol with (at
best) vaguely-defined semantics, including a requirement by the CA
to guess whether a particular request was a renewal or not
(updates were even more vaguely defined). In response to
developer feedback that they either avoided renewal/update
entirely because of this uncertainty or hardcoded in particular
behaviour on a per-CA basis, this specification explicitly
identifies renewal and update requests as such, and provides
proper semantics for both.
o Added the "SCEPStandard" keyword to GetCACaps to indicate that the
CA complies with the final version of the SCEP standard, since the
definition of what constitutes SCEP standards compliance has
changed significantly over the years.
o Removed the discussion in the security considerations of
revocation issues, since SCEP doesn't support revocation as part
of the protocol.
o Clarified the use of nonces, which if applied as originally
specified would have made the use of polling in the presence of a
lost message impossible.
o Removed the discussion of generating a given transactionID by
hashing the public key, since this implied that there was some
special significance in the value generated this way. Since it
was neither a MUST nor a MAY, it was unsound to imply that servers
could rely on the value being generated a certain way. In
addition it wouldn't work if multiple transactions as discussed in
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Section 4.4 were initiated, since the deterministic generation via
hashing would lead to duplicate transactionIDs.
o Added examples of SCEP messages to give implementers something to
aim for.
Appendix B. Sample SCEP Messages
(Omitted from the drafts to keep the size down).
Author's Address
Peter Gutmann
University of Auckland
Department of Computer Science
Auckland
New Zealand
Email: pgut001@cs.auckland.ac.nz
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